According to Eurogamer, the main issues the Counter-Strike pros had with the game were "a number of "feel" and operating issues" that Valve feels need to be resolved before opening the game up to an open beta.
Valve is adopting a "it's done when it's done" mentality to CS: GO, as Chet Faliszek stated, "And then when does it end? When the community tells us. It ends when it's ready. We have no mandate from anybody of when we have to ship this. So we're more than happy to just keep working on this 'til it's ready to ship."
Counter-Strike: Global Offensive was announced this past August for the PC, Xbox 360 and PlayStation 3. The beta, however, will be exclusively on the PC platform.